1. Field of the Invention
This invention relates to a numerical control system for a machine tool having a feed speed overriding unit and a main spindle rotational speed overriding unit, in particular, to a numerical control system for a machine tool achieving a satisfactory processing in a state controlled by a feed speed overriding unit and a main spindle rotational speed overriding unit.
2. Description of the Related art
When the work is being processed, for example, during a cutting operation, by a machine tool, the cutting blade of the machine tool intermittently abuts the work piece and makes it vibrate.
If the frequency of the vibration of the work piece caused by the blade of the tool is near the resonance frequency of the work piece, the vibration called chattering may be generated which may produce an unsatisfactory machining process.
During a machining process, if the frequency of the vibration of the blade of the tool is near the resonance frequency of the tool, the vibration called chattering may also be generated which may also produce an unsatisfactory machining process. This tends to happen if the tool is long.
In order to prevent the chattering and maintain a satisfactory machining processing, it is required to prevent the vibration of the work piece whose frequency is near the resonance frequency of the machining process or the tool. According to the prior art, this is achieved by decreasing the feed speed and the main spindle rotational speed.
However, the processing time increases if the feed speed and the main spindle rotational speed are decreased during the entire processing. Thus, the feed speed and the main spindle rotational speed are preferably decreased only in some cases out of necessity. A numerical control system is known which comprises a feed speed overriding unit and a main spindle rotational speed overriding unit, as a numerical control system for a machine tool for decreasing the feed speed and the main spindle rotational speed only in some cases depending upon the necessity. In such a control system, the feed speed overriding unit outputs a signal for an overriding control of the feed speed according to an input of a feed speed overriding, and the main spindle rotational speed overriding unit outputs a signal for an overriding control of the main spindle rotational speed according to an input of the main spindle rotational speed overriding.
In the conventional numerical control system as described above, the operator has to manually conduct the input of the feed speed overriding to the feed speed overriding unit and manually conduct the input of the main spindle rotational speed overriding to the main spindle rotational speed overriding unit, while supervising the machining.
For example, the operator conducts overriding inputs in such a manner that the feed speed and the main spindle rotational speed are increased when the machine process starts, and that the feed speed and the main spindle rotational speed are decreased when the resonance occurs and grows during the machine process.
In the overriding inputs, the feed speed and the main spindle rotational speed have to be changed synchronously with each other in order to maintain an even feed of each cutting blade.
However, it is difficult for the operator's manual inputs to change the feed speed and the main spindle rotational speed synchronously with each other, thus, it is difficult to achieve a good processing state. Therefore, the processed surfaces of the work may tend to be uneven. In addition, the operator is over burdened by conducting several manual inputs, so that the operator can only be assigned to work a limited number of machines.
On the other hand, special tools with higher stiffness are available in order to prevent the generation of the chattering. However, the special tools are very expensive.
Therefore, the object of this invention is to provide a numerical control system for a machine tool which can effectively prevent the resonance phenomenon when performing a machining process on a workpiece.